A fluid dynamic bearing device has been used for a cylinder bearing of video tape recorder etc. because the device has a feature that non-repeatable runout (NRRO) is small as compared with a ball bearing, and the device is suitable for high-speed rotation. On the other hand, in the magnetic disk device, a rotational accuracy of a conventional spindle motor using a ball bearing is insufficient in accordance with recent increase in magnetic recording density, and the spindle motor has been shifted to a spindle motor using a fluid dynamic bearing. Further, the fluid dynamic bearing has a feature that an operation sound is quiet under operating because there is no solid contact portion unlike a ball bearing.
Recently, the magnetic disk device has been widely used also in the field of a domestic electric product such as being built in a DVD recorder. In the field of a domestic electric product, there is a need to reduce an operation sound especially during operations of the product, and the use of the magnetic disk device equipped with a spindle motor that uses a fluid dynamic bearing has rapidly expanded. Further, the use of the magnetic disk device with a fluid dynamic bearing spindle motor as an external memory of a computer in a notebook or desktop type personal computer or an RAID device has expanded for similar reasons.
In general, the dynamic pressure fluid dynamic bearing device is composed of a stationary member and a rotational member. The stationary member includes a thrust fluid dynamic bearing and a radial fluid dynamic bearing, and a lubrication fluid is maintained between the rotational member and the stationary member. When the spindle motor is rotated, a lubrication fluid moves through a U-shaped, V-shaped, or herringbone groove formed in a thrust fluid dynamic bearing, a radial fluid dynamic bearing, etc., and a dynamic pressure is generated by the pumping action. As a result, the rotational member is raised from the stationary member and is kept in noncontact with the stationary member.
A friction resistance during rotation of the fluid dynamic bearing spindle motor is chiefly generated by viscosity of the lubrication fluid. In general, the temperature dependence of the viscosity of the lubrication fluid is high. In particular, a necessary start torque increases because the viscosity rises remarkably at the low temperature, and a voltage margin equivalent to that at the normal temperature might not be secured in the spindle motor driving system. Therefore, it takes much time to reach the rated rpm or the rated rpm is not reached, or a rotational failure occurs in some cases.
Moreover, the spindle motor is started in the magnetic disk unit that adopts the load/unload mechanism with a head stack assembly (HSA) being retracted. Afterwards, when the spindle motor reaches the rated rpm, the HSA is loaded onto a rotating disk. In this case, the drag torque is generated along with the loading of the HSA, which applies a further load onto the spindle motor.
To that end, a method of lowering the viscosity of a lubrication fluid is necessary to decrease the load upon starting the spindle motor at the low temperature, and the following measures have been taken as the conventional technique.
Japanese Patent Publication No. 2000-224891 (“Patent Document 1”) discloses a technique of supplying an overcurrent to a coil of a spindle motor to heat the spindle motor and reduce the viscosity of a lubrication fluid for the purpose of suppressing increase in rotational load under low-temperature operation environments.
Japanese Patent Publication No. 2006-134377 (“Patent Document 2”) discloses that a current is supplied to preheat a magnetic transducer before a spindle motor reaches a rated speed to ensure write characteristics under lower temperature of the magnetic transducer, and describes a sequence of loading an HSA after the elapse of a predetermined period to preheat the magnetic transducer after the rated speed is reached.
In recent years, there are more and more cases of installing a magnetic disk device with a spindle motor in notebook or desktop type personal computer. In addition, there are more and more opportunities that the device is used in the form of composing RAID as an external memory of a large computer system. Thus, the magnetic disk device is requested to operate in various use environments, and in particular, a demand to expand an operating temperature range has been increasing. Concretely, the operating temperature range of 5° C. to 55° C. is expanded to the operating temperature range from 0° C. to 60° C.
On the other hand, there is a tolerance also in a power supply voltage supplied to the magnetic disk device. In general, variations of a reference supply voltage of ±10% to ±5% are allowed and defined as specifications.
When the operating temperature is changed from 5° C. to 0° C., the viscosity of a lubrication fluid rises remarkably because a temperature dependence of the viscosity of the lubrication fluid of the fluid dynamic bearing spindle motor is high. In particular, if the magnetic disk device starts after being left in the atmosphere of 0° C. for long time under power-off, a problem that the voltage margin of the spindle motor driving system will be lost, and a rated speed cannot be reached, for instance, when a supply voltage is −10% (10.8 V to 12V) because the rotation load increases by the rise of the viscosity of the lubrication fluid.
Moreover, in the magnetic disk device that adopts the mechanism that load/unloads the magnetic head on the disk, the spindle motor is started with the HAS having a magnetic transducer unloaded and a rated speed is reached, and then a head slider is loaded on the rotating magnetic disk. Then the head slider is loaded, the drag torque is generated because a part of an arm of a carriage that constitutes the HSA that holds the head slider opposes the rotating magnetic disk surface, and the rotation load of spindle motor further increases. Therefore, if the spindle motor driving system cannot deal with an increase of the load upon loading the HSA, the spindle motor cannot maintain the rated speed. That is, the voltage margin will be lost in the unloaded state, and a rotation failure might occur even at the rated speed in the loaded state. The magnetic disk device can enter a usable state (READY) only after the spindle motor reaches the rated speed with the head slider loaded. It is not possible to respond to a host system connected to the magnetic disk device unless the device is READY.
On the other hand, in a host system such as a personal computer using the magnetic disk device as a data storage, a period from power-on of the host system until when the magnetic disk device becomes READY is previously determined. If READY cannot be sent back within the period preset by the host system because it takes much time for the spindle motor to reach a rated speed in the magnetic disk device, a timeout error occurs. Thus, the period from power-ON to READY is defined as specifications in the magnetic disk device.
Patent Document 1 above discloses that the spindle motor is heated to supply an overcurrent to a coil for reducing a viscosity of a lubrication fluid. However, under such conditions that the voltage margin is lost, the overcurrent hardly flows. Further, there is no description about increase in load to the spindle motor upon loading.
Patent Document 2 above describes that the HSA is loaded after a predetermined period from when the spindle motor reaches the rated speed, but a period necessary for heating a magnetic transducer is as short as several hundreds of mm, so an effect of heating the lubrication fluid of the spindle motor cannot be expected.